Deprecate HMP `cpu-add`

Since we're deprecating the QMP `cpu-add`, let's deprecate its HMP
equivalent, too.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Message-Id: <20181030123526.26415-3-kchamart@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
diff --git a/hmp.c b/hmp.c
index 7828f93..43ae9ec 100644
--- a/hmp.c
+++ b/hmp.c
@@ -2372,6 +2372,8 @@
     int cpuid;
     Error *err = NULL;
 
+    error_report("cpu_add is deprecated, please use device_add instead");
+
     cpuid = qdict_get_int(qdict, "id");
     qmp_cpu_add(cpuid, &err);
     hmp_handle_error(mon, &err);